The present invention relates to fuel metering systems, and more specifically, to a fuel metering system having multiple valves to reduce P1-P2 variations and thus improve metered flow accuracy.
A conventional fuel metering system includes a metering valve, and single stage servo valve, and a pressure regulating valve (whose job is to regulate metering window pressure (P1-P2) by bypassing excess pump flow), and a servo pressure regulating valve (whose job is to regulate servo pressure (PR-PD) across the single stage servo valve). Because of the large variations in bypass flow and system delta pressure (P1-PD), the regulation of P1-P2 can have significant variation. This variation is largely due to a large valve stroke introducing large spring force variations, and the large swing in P1-PD causing large changes in fluid momentum forces. As such, there can be large variations in the pressure P1-P2, resulting in inaccuracies in metered flow within the system. In addition, the conventional metering valve has several signal pressures making it a very large and complex valve.